The present invention relates generally to information displays and, more particularly, to apparatus for converting from any of several input scan formats to a fixed output format.
Information displays are rapidly converting from cathode ray tube (CRT) technology to various forms of solid-state driven matrix displays. While CRT's can easily be scanned with various formats simply by changing the vertical and horizontal scan frequencies, matrix displays have a fixed geometrical format. Matrix displays can easily be scanned at different vertical frequencies, but the number of lines in the vertical direction, and the aspect ratio of the image, are both fixed.
As a consequence of the widespread use of computers and many television formats, there is wide range of scan formats ranging from 262 lines to 2,048 lines. Vertical scan frequencies range from 50 to 72 Hz, scan formats are both progressive and interlaced, and aspect ratios vary from 16:9 to 3:4. Accordingly, if one uses a fixed pattern CCD television camera, for example, and wishes to present the video information on a display having a fixed format different from that of the camera, it is necessary to scan convert. While it is fairly easy to operate either the camera or the display at any desired vertical scan rate, such as 59.94 Hz, 60 Hz or 50 Hz., there is a problem should it be necessary to use a different number of lines than the device was made for, be it camera or display, because of their fixed physical pattern.
Interpolation has long been successfully used for scan conversion, and usually involves interpolation between two adjacent lines with the appropriate ratios for the derived output line. An example of such known apparatus is the line-rate converter described in the article entitled "An HDTV Down-Converter for Post-Production", L. Thorpe et al, SMPTE Journal, February 1990, pp 124-135, in which the 525-line rate required at the output is synthesized from the higher 1125-line rate of the HDTV input. The HDN-2000 down-converter described in this article is an intra-field line-rate converter that uses a frame store and a moderately sophisticated interpolation scheme. Breaking the 1125-to-525 conversion ratio down to its simplest form produces the ratio 15:7, indicating that the line interpolation process requires fifteen lines of HDTV video to synthesize seven output lines of 525-line video. Within this structure, the line interpolation process becomes a decision about how many adjacent HDTV input lines will be employed to structure a single output line of video. The hardware implementation of the interpolation consists of a series of one-line delay elements that allow simultaneous access to each of the requisite number of HDTV input lines; the HDN-2000 used four HDTV input lines to construct a given 525 output line, which gives better performance than if only two lines are used for the interpolation routine.
It is known from the article entitled "Signal Processing for Compatible HDTV", W. E. Glenn and K. Glenn, SMPTE Journal, November 1989, pp. 812-816, that interline flicker occurs with interlaced scans due to the residual low spatial frequencies that are displayed with counterphase flicker at the frame rate. Among the circuits discussed for solving the interline flicker problem is the signal processor used in an IDTV receiver in which 525 lines can be displayed progressively by scan conversion from the 525-line interlaced NTSC transmission. A high-pass digital spatial filter passes information for the top octave of the signal vertically and horizontally, which information is stored in a frame store. The low spatial-frequency information is derived from the current field of the interlaced transmission. High- and low-frequency information is combined and read out progressively at 60 frame per second.
These are but two examples of known scan converters which use a frame store in which the input signal is stored and then taken out at a different line rate by performance of a suitable interpolation routine. Such use of frame stores is premised on the theory that the vertical resolution in a camera is one frame line high and that, therefore, a frame store must be used when converting, say, from an interlaced input to a progressive output. Actually, the vertical resolution in most cameras is two lines high so that there is little, if any, advantage in using a frame store in converting from interlaced to progressive; indeed, the use of a frame store has the disadvantage that some information is delayed by a field behind other information which causes undesirable smear in moving objects, which has to be circumvented with adaptive processing which may include motion detection, all of which adds to the complexity and cost of the scan converter.
A primary object of the present invention is to provide an adaptive scan converter which is relatively simple and sufficiently low in cost to enjoy widespread acceptance.
Another object of the invention is to provide a scan converter having the capability of converting from any of several input scan formats to a fixed output format.
Other objects of the invention are to provide a scan converter which automatically provide the correct aspect ratio; and which provides a progressive display with good motion rendition and good vertical resolution, without interline flicker, regardless of whether the input is interlaced or progressive.